Recently developed work machines include electric motors and electricity storage devices (for example, batteries and electric double-layer capacitors) for more enhanced energy efficiency and better energy saving than in conventional work machines that employ hydraulic actuators. A known hydraulic excavator as a type of such work machines includes an electric motor in place of a conventional hydraulic motor for a swing actuator that drives and swings an upper swing structure with respect to a lower track structure (see, for example, patent document 1).
Another known arrangement includes a hydraulic motor and an electric motor as the swing actuator to drive the swing structure through combined torque of the hydraulic motor and the electric motor (see, for example, patent document 2).
Kinetic energy of the swing structure during deceleration (braking) of the swing actuator is abandoned as heat on a hydraulic circuit with the conventional hydraulic motor. With the conventional electric motor, however, the kinetic energy is expected to be regenerated as electric energy and thus greater energy saving can be achieved.
When an electric motor performs powering or regeneration, heat can be generated for causes of electrical resistance and friction. A relatively low heating value can be dissipated by a cooling device; however, a high heating value may not be sufficiently dissipated by the cooling, which causes the electric motor to become hot, resulting at times in fusion or other fault.
Patent document 1 discloses a technique that incorporates a temperature sensor for detecting temperature of coolant for cooling an inverter. When the coolant temperature is high, the technique controls and reduces an upper limit value of a current supplied to an electric motor, thereby preventing the temperatures of the electric motor and the inverter from rising inordinately.
Patent document 2 discloses a technique that incorporates temperature sensors for an electric motor and inverter. The technique controls and reduces output of the electric motor in accordance with an increase in temperatures, thereby preventing the temperatures of the electric motor and the inverter from rising inordinately.